The present invention relates to extruding of elastomeric polymer compositions, more specifically to such compositions that are extruded as films for use in disposable hygiene products such as adult, infant, and feminine hygiene products.
Extrudable elastomeric compositions which can be easily made into elastic films having low stress relaxation, low hysteresis, and high recoverable energy are described in U.S. Pat. Nos. 4,663,220, 4,789,699, 4,970,259, or 5,093,422. Those elastomeric films are useful in making a variety of applications such as diaper waistbands and non-woven fabrics.
Polystyrene-poly(ethylene-butylene)-polystyrene elastomeric block copolymers (SEBS) and polystyrene-poly (ethylene-propylene)-polystyrene (SEPS) elastomeric block copolymers have been blended with other materials such as, for example, polyolefins, such as polypropylene and polyethylene, and oils to form extrudable elastomeric compositions which can be more easily extruded into elastic films having improved processing properties. While the additives improve the extrusion properties of the compositions and the processing properties of the elastic films, such additives have an adverse affect on the elastic properties of the resulting film, especially at temperatures above 25xc2x0 C.
The currently used commercial SEBS-based compounds for elastomeric films generally have an average stress relaxation, when measured on a film in the direction that is transverse to the machine direction and tested at 100xc2x0 F. (37.8xc2x0 C.) at 150 percent elongation, of greater than 27 percent, and a retained tension or load of less than 105 psi. Measurement in the transverse direction is the most important because when the film is assembled into the final product, it is the direction that gets stressed. Since these films are mainly used in disposable hygiene products, they have to be able to retain their properties at body temperature when stressed. Stress relaxation refers to the percent loss of tension or load encountered after elongating an elastic material at a specified rate of extension to a predetermined length and is expressed as a percentage loss of the initial load encountered at a specified extension of the elastic material. For this application, the lower the stress relaxation the better, and higher retained tension or load, i.e. above 125 psi., will allow the use of thinner films.
The present invention is an improved block copolymer composition for extruding films having excellent stress relaxation and increased retained tension or load at elevated temperatures. The composition is comprised of:
(a) from 52 to 60 percent by weight of a block copolymer having at least two polystyrene endblocks and a midblock of hydrogenated polymerized butadiene (SEBS), having a polystyrene content of 14 to 25 percent by weight, a polystyrene block number average molecular weight of 7,000 to 11,000, a butadiene block number average molecular weight of 70,000 to 90,000, and a vinyl content of 45 percent by weight or less,
(b) from 13 to 22 percent by weight of polystyrene, and
(c) from 19 to 28 percent by weight of oil.
The extrudable elastomeric composition of the present invention is an improvement of the extrudable compositions described in U.S. Pat. Nos. 4,970,259 and 5,093,422 which descriptions are incorporated by reference herein. The known compositions include one or more styrenic block copolymers, typically a polystyrene-poly(ethylene-butylene)-polystyrene (S-EB-S) or a polystyrene-poly(ethylene-propylene)-polystyrene (S-EP-S) elastomeric block copolymer which is produced by hydrogenating a polystyrene-polybutadiene-polystyrene or polystyrene-polyisoprene-polystyrene block copolymer. The extrudable compositions further comprise a polyolefin and an extending oil.
The styrenic block copolymers have at least two poly(monoalkenyl arene) blocks, preferably two polystyrene blocks, separated by a saturated block of polybutadiene (EB). The SEBS block copolymers of this invention comprise polystyrene endblocks each having a number average molecular weight of 7,000 to 11,000 and saturated polybutadiene midblocks having a number average molecular weight of 70,000 to 90,000. The saturated polybutadiene blocks must have a vinyl content of 45 percent by weight or less and a polystyrene content of 14 to 25 percent by weight.
The term xe2x80x9cvinyl contentxe2x80x9d refers to the fact that a conjugated diene is polymerized via 1,2-addition (in the case of butadienexe2x80x94it would be 3,4-addition in the case of isoprene). Although a pure xe2x80x9cvinylxe2x80x9d group is formed only in the case of 1,2-addition polymerization of 1,3-butadiene, the effects of 3,4-addition polymerization of isoprene (and similar addition for other conjugated dienes) on the final properties of the block copolymer will be similar. The term xe2x80x9cvinylxe2x80x9d refers to the presence of a pendant vinyl group on the polymer chain.
These polymers may be prepared using free-radical, cationic and anionic initiators or polymerization catalysts. Such polymers may be prepared using bulk, solution or emulsion techniques. In any case, the polymer containing at least ethylenic unsaturation will, generally, be recovered as a solid such as a crumb, a powder, a pellet, or the like.
In general, when solution anionic techniques are used, conjugated diolefin polymers and copolymers of conjugated diolefins and alkenyl aromatic hydrocarbons are prepared by contacting the monomer or monomers to be polymerized simultaneously or sequentially with an organoalkali metal compound in a suitable solvent at a temperature within the range from about xe2x88x92150xc2x0 C. to about 300xc2x0 C., preferably at a temperature within the range from about 0xc2x0 C. to about 100xc2x0 C. Particularly effective anionic polymerization initiators are organolithium compounds having the general formula:
RLin
wherein R is an aliphatic, cycloaliphatic, aromatic, or alkyl-substituted aromatic hydrocarbon radical having from 1 to about 20 carbon atoms; and n is an integer of 1 to 4.
In addition to sequential techniques to obtain triblocks, at least anionic initiators can be used to prepare diblocks of polystyrene-polydiene having a reactive (xe2x80x9clivexe2x80x9d) chain end on the diene block which can be reacted through a coupling agent to create, for example, (S-B)x Y structures wherein x is an integer from 2 to about 30, Y is a coupling agent, B is butadiene and greater than 65 percent of S-B diblocks are chemically attached to the coupling agent. Y usually has a molecular weight which is low compared to the polymers being prepared and can be any of a number of materials known in the art, including halogenated organic. compounds; halogenated alkyl silanes; alkoxy silanes; various esters such as alkyl and aryl benzoates, difunctional aliphatic esters such as dialkyl adipates and the like. Depending on the selected coupling agent the final polymer can be a fully or partially coupled linear triblock polymer (x=2). The coupling agent, being of low molecular weight, does not materially affect the properties of the final polymer.
It is not required in coupled polymers that the diblock units all be identical. In fact, diverse xe2x80x9clivingxe2x80x9d diblock units can be brought together during the coupling reaction giving a variety of unsymmetrical structures, i.e., the total diblock chain lengths can be different, as well as the sequential block lengths of styrene and diene.
The styrenic block copolymers must be hydrogenated. In general, the hydrogenation or selective hydrogenation of the polymer may be accomplished using any of the several hydrogenation processes known in the prior art. For example the hydrogenation may be accomplished using methods such as those taught, for example, in U.S. Pat. Nos. 3,494,942; 3,634,594; 3,670,054; 3,700,633; and Re. 27,145, the disclosure of which patents are incorporated herein by reference. The methods known in the prior art and useful in the present invention for hydrogenating polymers containing ethylenic unsaturation and for hydrogenating or selectively hydrogenating polymers containing aromatic and ethylenic unsaturation, involve the use of a suitable catalyst, particularly a catalyst or catalyst precursor comprising an iron group metal atom, particularly nickel or cobalt, and a suitable reducing agent such as an aluminum alkyl.
In general, the hydrogenation will be accomplished in a suitable solvent at a temperature within the range from about 20xc2x0 C. to about 100xc2x0 C. and at a hydrogen partial pressure within the range from about 100 psig to about 5,000 psig, preferably about 100 psig. to 1,000 psig. Catalyst concentrations within the range from about 10 ppm (wt) to about 500 ppm (wt) of iron group metal based on total solution are generally used and contacting at hydrogenation conditions is generally continued for a period of time within the range from about 60 to about 240 minutes. After the hydrogenation is completed, the hydrogenation catalyst and catalyst residue will, generally, be separated from the polymer.
The compound of this invention contains from 52 to 60 percent by weight of the above block copolymer. It is important that the total amount of polymer in the compound be between 52 to 60 percent by weight of the compound. If this condition is not satisfied, then the compound will have stress relaxation values greater than 27 percent when the polymer content is below 52 percent and the compound will not be processable into thin films (0.001-0.020 inches thick) when the polymer content is above 60 percent.
The compound should contain from 13 to 22 percent by weight of polystyrene because it is compatible with the styrene end blocks of the polymer, which helps to increase the retained tension or load, and it also aids in the processing of the final compound. The polystyrene used in the compound of the present invention may be one which has a number average molecular weight of from 150,000 up to about 400,000, most preferably 200,000 to 300,000, and a melt flow index from 1 to 18, preferably 1 to 7, ASTM D1239, condition G.
The amount of oil used in the compound ranges from 19 to 28 percent by weight because it aids in the processing of the final compound and helps reduce the amount of stress relaxation. If the oil content is not within this range, then the stress relaxation will be greater than 27 percent when the percentage of oil is less than 19 percent and the retained tension or load will be below 105 psi when greater than 29 percent. Oils which can be used are those which are compatible with the elastomeric mid-block segment of the elastomeric block copolymer and which do not tend to go into the aromatic endblock portions to any significant degree. Thus, the oils can be viewed as paraffinic. Paraffinic oils which may be used in the elastomeric composition should be capable of being melt processed with other components of the elastomeric composition without degrading. Particularly important is the ability of the final composition to be melt extruded. An exemplary extending oil is a white mineral oil available under the trade designation Drakeol 34 from the Pennzoil Company Pennreco Division. Drakeol 34 has a specific gravity of 0.864-0.878 at 60xc2x0 F., a flash point of 460xc2x0 F., and viscosity of 370-420 SUS at 100xc2x0 F. Suitable vegetable oils and animal oils or their derivatives may also be used as the extending oil.
While the principal components of the extrudable elastomeric composition used to form the elastic sheet have been described in the foregoing, such extrudable elastomeric composition is not limited thereto, and can include other components not adversely affecting the extrudable elastomeric composition attaining the stated objectives. Exemplary materials which could be used as additional components would include, without limitation, pigments, antioxidants, stabilizers, surfactants, waxes, flow promoters, solvents, particulates, and materials added to enhance processability and pellet handling of the composition.
The term xe2x80x9celasticxe2x80x9d is used herein to mean any material which, upon application of a biasing force, is stretchable, that is, elongatable to at least 400 percent of its relaxed unbiased length and which will recover to within 140 percent of its original unbiased relaxed length upon release of the stretching, elongating force. A hypothetical example would be a one (1) inch sample of a material which is elongatable to at least 5.00 inches and which, upon being elongated to 5.00 inches and released, will recover to a length of not more than 1.40 inches. Many elastic materials may be elongated by much more than 400 percent of their relaxed length, for example, elongated 400 percent or more, and many of these will recover to substantially their initial relaxed length, for example, to within 140 percent of their initial relaxed length, upon release of the stretching force.
As used herein, the term xe2x80x9cstress relaxationxe2x80x9d refers to the percent loss of tension or load between the maximum load or force encountered after elongating an elastic material at a specified rate of extension to a predetermined length (or the load or force measured at some initial length) and the remaining load or force measured after the sample has been held at that length for a specified period of time, for example, from about 0 minutes to about 60 minutes. Except where otherwise noted for the present invention, stress relaxation is expressed as a percentage loss of the initial load encountered at a specified extension of an elastic material. The stress relaxation is determined by calculating the difference between the initial maximum load measured after elongating an elastic material in the transverse direction at a rate of 20 inches per minute to an elongation of 150 percent (i.e., to 150 percent of the material""s initial width) and the remaining load measured after that sample was held at that width for 60 minutes divided by the initial maximum load at that width. Testing may be performed on an Instron Model 5565 Universal Test Machine specimens based on ASTM D882. Stress relaxation after 60 minutes at, for example, an elongation of 150 percent (i.e., to 150 percent of the material""s initial width) may be expressed as a percentage utilizing the following equation:
stress relaxation=(peak load150%xe2x88x92load150%@ 60 min)/(peak load150%)*100.
As used herein, the xe2x80x9cnumber average molecular weightxe2x80x9d was determined utilizing gel permeation chromatography techniques for linear anionic polymers. All molecular weights are measured prior to hydrogenation which will increase the molecular weights by a small amount.
As used herein, the polystyrene content of a block copolymer refers to the weight of polystyrene in the block copolymer. It is calculated by dividing the sum of molecular weight of all polystyrene blocks by the total molecular weight of the block copolymer.